GC Using LC Columns

Micropacked columns for LC can be applied to GC separation of hydrocarbons. When a glass-lined stainless-steel tube of 30 cm x 0.3 mm i.d., packed with 5 |im alkyl-modified silica, is employed as the separation column, a microvalve injector for LC should be used because of the large pressure drop across the column.

Figure 6 demonstrates the separation of C6-C20 straight-chain hydrocarbons using carbon dioxide carrier gas, an octadecylsilica (ODS) column and an FID. The inlet pressure of carbon dioxide as the carrier gas is kept at 6.2 MPa. A volume of 0.02 |L of the sample dissolved in pentane is injected. The concentration is c. 2% (v/v) each, corresponding to c. 0.3 |g each of the injected amount. The column temperature is 35°C for the initial 5 min, programmed to 230°C at a rate of 5°C min-1, and then held at 230°C. As far as alkyl-modified silica packings are employed, the SFC mode will be favoured for hydrocarbons with a carbon number larger than 15 because an increase in the background signal of the FID is observed at temperatures higher than 160°C.

Figure 6 Separation of an artificial mixture of straight-chain hydrocarbons. Column: Capcell Pak C18 (5 |im ODS), 300 x 0.3 mm i.d. Mobile phase: carbon dioxide. Inlet pressure: 6.2 MPa. Column temperature: 35°C for the initial 5 min, then programmed at 5°C min-1, held at 230°C. Peaks: straight-chain hydrocarbons (the numbers refer to the carbon numbers of the corresponding straight-chain hydrocarbons), c. 2% (v/v) each dissolved in pen-tane is injected. Detector: FID. (Reproduced with permission from Takeuchi et al. (1989). New approach to the GC separation of hydrocarbons by using LC-like microcolumns. Chromatographia 27: 183.)

Figure 6 Separation of an artificial mixture of straight-chain hydrocarbons. Column: Capcell Pak C18 (5 |im ODS), 300 x 0.3 mm i.d. Mobile phase: carbon dioxide. Inlet pressure: 6.2 MPa. Column temperature: 35°C for the initial 5 min, then programmed at 5°C min-1, held at 230°C. Peaks: straight-chain hydrocarbons (the numbers refer to the carbon numbers of the corresponding straight-chain hydrocarbons), c. 2% (v/v) each dissolved in pen-tane is injected. Detector: FID. (Reproduced with permission from Takeuchi et al. (1989). New approach to the GC separation of hydrocarbons by using LC-like microcolumns. Chromatographia 27: 183.)

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Figure 7 Separation of components of a kerosene. Operating conditions as in Figure 6, except for the sample and the temperature-programming rate. Column temperature: 35°C for the initial 5 min, then programmed at 3°C min-1. Sample: kerosene diluted twice with pentane. (Reproduced with permission from Takeuchi etal. (1989). New approach to the GC separation of hydrocarbons by using LC-like microcolumns. Chromatographia 27: 184.)

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Figure 7 Separation of components of a kerosene. Operating conditions as in Figure 6, except for the sample and the temperature-programming rate. Column temperature: 35°C for the initial 5 min, then programmed at 3°C min-1. Sample: kerosene diluted twice with pentane. (Reproduced with permission from Takeuchi etal. (1989). New approach to the GC separation of hydrocarbons by using LC-like microcolumns. Chromatographia 27: 184.)

Figure 7 demonstrates the separation of the components of a kerosene. The temperature-programming rate of the column is reduced to 3°C min-1. The numbers in the figure correspond to the carbon numbers of straight-chain hydrocarbons. The results offer encouragement for the use of a micropacked column as a common separation column for GC, SFC and LC.

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